Oscilloscopes and other types of equivalent-time sampling systems are used to reconstruct the waveforms of many types of optical and electrical signals. Within an equivalent-time sampling system, samples of an applied signal are acquired by a sampler. A timebase within the equivalent-time sampling system establishes the timing of acquired samples, so that a waveform of the applied signal can be reconstructed on a display or other output device. Timing accuracy and jitter are performance parameters of the timebase that determine how accurately the waveform reconstructed by the sampling system represents the applied signal.
A first type of sampling system is shown in FIG. 1. The timebase for this sampling system includes a programmable delay that is relied upon to establish the timing of samples acquired by a sampler relative to an applied trigger signal. As a result, performance of the sampling system is limited by the performance attributes of the programmable delay. Due to inherent noise and inaccuracies of presently available devices and elements used to implement the programmable delay, timing accuracy of the sampling system is limited to approximately four picoseconds, and jitter is limited to greater than approximately one picosecond. This low timing accuracy and high jitter prevent this type of sampling system from accurately reconstructing the waveforms of many types of applied signals, such as those present in high data-rate communication systems.
A second type of sampling system is shown in FIG. 2. The timebase for this sampling system uses quadrature sampling of a clock signal to establish the timing of acquired samples of an applied input signal, and relies on the clock signal being synchronous with, or having an established periodic relationship with, the input signal. The quadrature sampling provides high timing accuracy (approximately 200 femtoseconds) and low jitter (approximately 100 femtoseconds). However, the acquired samples are timed according to a reference signal, which prevents the timing of acquired samples from being arbitrarily designated, thereby reducing the flexibility of this sampling system for reconstructing selected portions of the applied input signal. In addition, the sampling system is not suitable for reconstructing those portions of the applied input signal that do not lie within a single cycle, or period, of the clock signal.
In view of the shortcomings of the sampling systems shown in FIGS. 1-2, there is a need for a timebase that provides a sampling system with high timing accuracy and low jitter, and that also enables the timing of sample acquisitions to be arbitrarily designated.
A timebase constructed according to a first embodiment of the present invention establishes the timing of samples acquired by a signal sampler relative to a trigger signal that is synchronous with a signal applied to the signal sampler. The timebase has high timing accuracy, low jitter, and enables the timing of sample acquisitions of an applied signal to be arbitrarily designated.
A first pair of samplers included in the timebase acquires samples of a reference signal and of a shifted version of the reference signal provided within the timebase, according to a synchronous trigger, to establish a first time position on the reference signal. A second pair of samplers included in the timebase acquires samples of the reference signal and the shifted reference signal according to the synchronous trigger as delayed by a programmed time interval, to establish a second time position on the reference signal. While the programmed time interval is adjusted to designate timing of the sample acquisitions by the signal sampler, the two pairs of samplers in conjunction with a timing analyzer accurately determine the timing of these sample acquisitions based on the established time positions on the reference signal. As a result, timing accuracy and jitter of the timebase are independent of noise and inaccuracies of delay elements or devices used to set the programmed time interval. In an alternative embodiment of the present invention, the timebase is implemented according to a method that determines the timing of the samples acquired from the signal sampler.